Today, large quantities of plastic products composed of polyethylene, polypropylene, polystyrene, polyethylene terephthalate, polyvinyl chloride or the like are used and waste disposal thereof has become one of the main issues of environmental problems. More specifically, the waste disposal at present is incineration disposal or landfill disposal, and incineration disposal of polyethylene, for example, causes an incinerator to damage and shorten its life owing to large heat of combustion of polyethylene. Further, incineration disposal of polyvinyl chloride causes toxic gases to be generated.
On the other hand, in the case of landfill disposal of plastic products, the land suited for that purpose is limited. Moreover, when these plastic products are abandoned in natural environment, these will remain almost semipermanent because degradation thereof by microorganism and the like will hardly happen owing to their very high chemical stability. Consequently, the plastic products subjected to landfill disposal have caused spectacular sights to deteriorate or have become a cause of a problem such as pollution of life environment of marine organism.
Under such circumstances, nowadays, attention has been drawn to biodegradable plastics or plastics degradable under natural environment. Biodegradable plastics are known to be gradually collapsed or decomposed in soil or water through hydrolysis or biodegradation and finally changed into harmless decomposed materials by the action of microorganisms. The biodegradable plastics under investigation for practical use today are roughly classified into plastics of natural materials composed mainly of biocelluloses or starch; aliphatic polyesters; modified polyvinyl alcohols; cellulose ester compounds; modified materials of starch; and blends thereof. Among these biodegradable plastics, aliphatic polyesters can be listed as well balanced and easy to apply to various uses in view of processability, cost, mechanical characteristics, water resistance and the like.
As aliphatic polyesters, for example, poly(hydroxybutyric/valeric) acid is known as a microorganism-produced polymer; polycaprolactone or a condensate of an aliphatic dicarboxylic acid and an aliphatic diol is known as a synthesized polymer; and a polylactic acid polymer is known as a semisynthesized polymer.
The polylactic acid polymer is synthesized from nonpetroleum raw materials such as sweet potato and corn, and is drawing public attention as a plant originated polymer which does not use petroleum resource, and, in the field where petroleum originated plastics have been exclusively used so far, the petroleum originated plastics have a trend of being vigorously replaced by polylactic acid polymers.
The polylactic acid polymer is mainly used in films or sheets by making the most of its transparency. However, the polylactic acid polymer alone, owing to its low heat resistance, can hardly replace conventional petroleum originated plastics such as transparent polyvinyl chloride or polyethylene terephthalate.
There are examples in which polycaprolactone or an inorganic filter is added to the polylactic acid polymer to improve its heat resistance, however, the excellent transparency of the polylactic acid polymer disappears in compensation for the improvement of its heat resistance.
Further, there are investigations in which an impact strength modifier is added to the polylactic acid polymer to improve its impact strength (refer to Patent Documents 1 to 2). In these cases, the excellent transparency of the polylactic acid polymer disappears, though its impact resistance is improved.    Patent Document 1: Japanese Patent No. 2,725,870    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2003-286396